Current multiphase flow meters do not achieve good results when the gas ratio of a multiphase flow becomes significant.
This is illustrated by the trumpet shape curve shown in FIG. 1 that shows a graphic representing the typical relative error rate of the measured values of liquid (or oil or water) fraction flow as a function of the gas volume fraction (GVF). The curve shows that for gas volume fraction (GVF) greater than 85%, the relative error rate becomes greater than about 5% and becomes unpredictable when the gas volume fraction (GVF) becomes greater than 95%.
One of the known techniques to address this issue is to separate a large quantity of gas from the multiphase flow upstream of the flow meter and therefore getting a lower ratio of gas in the flow. Then it becomes possible to get a significant accuracy for each of the three phases. Due to low pressure at surface condition, gas is more present in the multiphase flow than any other phases. Thus, separating gas from liquid phase requires the use of bulky equipment set upstream of the multiphase meter. This is inconsistent to the target for which multiphase flow meters were devised which was to eliminate the separator and of the complexity of use thereof.
Another way of dealing with the problem of accuracy of the measurement of the flow of the different phase is to make a partial separation of the gas from a main line of oil, water and gas using a gas diversion line. Some of the gas is diverted from the main line upstream of the multiphase meter but it is important to mention that still some gas is flowing through the main line of the multiphase meter.
This partial separation requires a gas to be substantially perfectly dry to avoid losing some of the liquid through the gas diversion line. The consequence is that in this type of configuration, the issue of gas quality (entrainment of liquid inside the gas) becomes as problematic as with a normal separator and the same problems of carry over or carry under are present, these two phenomena being related to a poor separation of the gas and liquid phases due either to one predominant phase flowing fast or lack of contrast of density, such as in Heavy Oil leading in the first case (i.e., carry over), to have some droplets going through the gas line, and in the second case (i.e., carry under) some bubbles going through the oil or water line. To cope with this issue, some manufacturers include additional sensors to control or measure the dryness of the extracted gas. Obviously, this inclusion of an additional sensor to measure or control dryness does not lead toward reduction and simplification of the multiphase flow meters. It leads to complexity and higher cost.
Another way to handle important error rate in multiphase flow measurements is to provide upstream of the multiphase flow meter a retention section for loading the multiphase flow meter with some liquid and release it in a controlled manner with the gas in a certain proportion to be able, first to reduce the relative gas content and second to get a better accuracy on the liquid flow rate from the multiphase flow meter. For example, Framo Engineering, and AGAR Corporation are among the first companies who designed respectively mixer (or retention) device and specific separation device upstream of the multiphase meter to handle this type of situation. As mentioned previously this leads to costly and bulky equipment.
In addition it is to be noted that the equipment to divert gas from the multiphase flow or for adding liquid phase to the multiphase flow require the separation or conditioning equipment to be designed for the expected flow rate. Obviously, the expected flow rate cannot be guaranteed for the entire life of a well, which may be over 20-50 years.
The market of multiphase flow meters is segmented into three main types of multiphase flow meters, a wet gas multiphase flow meter known as Type I flow meter, for measuring only the gas phase flow of the multiphase flow, multiphase flow meters known as type II capable to measure liquid and gas, and multiphase flow meters type III capable to measure and distinguish the 3 phases, i.e., oil or condensate, water, and gas.
In spite of the market segmentation, it is fundamentally the same accuracy of measurement problem present in all flow meter types. This issues is usually present at high Gas Volume Fraction (GVF) and therefore it is the same pathology in terms of accuracy, which is provided by any meters in the full range of GVF and water liquid ration (or ratio) (WLR).